The catalysts for the hydrotreating of hydrocarbon feedstocks to which the present invention relates are used under conditions appropriate for converting organosulphur compounds to hydrogen sulphide in the presence of hydrogen, which operation is known as hydrodesulphurization (HDS), and for converting organonitrogen compounds to ammonia in an operation which is known as hydrodenitrogenation (HDN).
These catalysts are generally based on metals from groups VIB and VIII of the Periodic Classification of the Elements, such as molybdenum, tungsten, nickel and cobalt. The most commonly used hydrotreating catalysts are formulated from cobalt-molybdenum (Co--Mo), nickel-molybdenum (Ni--Mo) and nickel-tungsten (Ni--W) systems on porous inorganic supports, such as aluminas, silicas or silicas/aluminas. These catalysts, manufactured industrially in very large tonnages, ate supplied to the user in their oxide forms (for example, cobalt oxides-molybdenum oxide catalysts on alumina, symbolized by the abbreviation: Co--Mo/alumina).
However, these catalysts are active in hydrotreating operations only in the form of metal sulphides. This is why, before being used, they have to be sulphurized.
As regards the activation of hydrotreating catalysts, the sulphurization of these catalysts is an important stage in obtaining their maximum performance with regard to HDS and HDN. As is indicated by the authors of Hydrotreating Catalysis (Catalysis, Vol. 11, 1996, p. 25, edited by J. R. Anderson and M. Boudart), practical experience has shown that the sulphurization procedure can have a significant influence on the activity and stability of the catalyst, and much effort has been devoted to improving the sulphurization procedures.
The most direct method for the sulphurization of a catalyst consists in treating the latter with hydrogen sulphide mixed with hydrogen. This method, which has formed the subject-matter of many patents (U.S. Pat. Nos. 3,016,347, 3,140,994, GB 1,309,457, U.S. Pat. Nos. 3,732,155, 4,098,682, 4,132,632, 4,172,027, 4,176,087, 4,334,982, FR 2,476,118), is generally only practised at the laboratory stage, as the use of hydrogen sulphide has major disadvantages which do not allow it to be employed on every industrial site.
The industrial procedures for the sulphurization of the catalysts are generally carried out under hydrogen pressure with liquid feedstocks already comprising sulphur compounds as sulphurizing agents. The method chiefly used in the past by refiners consisted in sulphurizing the catalysts with the sulphur-comprising oil feedstocks, but this technique had significant disadvantages because of the difficulty of converting the sulphur compounds to hydrogen sulphide. To avoid the reduction of the catalysts by the hydrogen, the sulphurizations, initiated at low temperature, had to be taken slowly to high temperature in order to obtain complete sulphurization of the catalysts.
Sulphur-comprising additives have been proposed for improving the sulphurization of the catalysts. The method consists in incorporating a sulphur compound (spiking agent) in a feedstock, such as a naphtha, or in a specific fraction, such as a VGO (vacuum gas oil) or an LGO (light gas oil). U.S. Pat. No. 3,140,994 was the first to claim the use of compounds of different natures which are liquid at ambient temperature: carbon disulphide, thiophene, mercaptans, dialkyl disulphides and diaryl disulphides. organic sulphides, in particular dimethyl sulphide, have also formed the subject-matter of claims. Dimethyl disulphide (DMDS) has been more particularly recommended for the sulphurization of the catalysts and an effective method for sulphurization with dimethyl disulphide is disclosed in Patent EP 64,429.
H. Hallie (Oil and Gas Journal, Dec. 20, 1982, pp 69-74) has reviewed these procedures for sulphurization under hydrogen which are carried out directly in hydrotreating reactors. These various techniques for the sulphurization of catalysts, known as "in situ" techniques, have been compared and studies have shown that sulphurization with a liquid feedstock to which has been added a sulphurizing agent which has the property of decomposing at low temperature (spiked feedstock) is the best sulphurization technique. The technique without an additional sulphurizing agent (nonspiked feedstock) gives a less active sulphurized catalyst. The sulphurizing agent which it is preferred to add to the feedstock is dimethyl disulphide.
Organic polysulphides have also been claimed as sulphurizing agents for the sulphurization of the catalysts. U.S. Pat. No. 4,725,569 discloses a method for the use of organic polysulphides of RS.sub.x R' type (it being possible for R and R' to be identical or different, with x equal to or greater than 3) which consists in impregnating the catalyst at ambient temperature with a solution comprising the polysulphide, in subsequently removing the inert solvent and, finally, in carrying out the sulphurization, under hydrogen, of the charged catalyst in the hydrotreating reactor. In Patent EP 298,111, the polysulphide of RS.sub.x R' type, diluted in a liquid feedstock, is injected during the sulphurization of the catalyst in the presence of hydrogen.
Functionalized mercaptans, such as mercaptocarboxylic acids or esters, dithiols, aminomercaptans and hydroxymercaptans, as well as thiocarboxylic acids or esters, are claimed in Patent EP 289,211 for the sulphurization of the catalysts.
More recently, new techniques for the sulphurization of the catalysts comprising two stages have been developed. In a first stage, known as an "ex situ" stage, the catalyst is preactivated in the absence of hydrogen outside the refinery after having been impregnated with a sulphurizing agent. The complete sulphurization of the catalyst is carried out in the hydrotreating reactor in the presence of hydrogen. The "ex situ" presulphurization relieves the refiner from injecting a sulphurizing agent during the sulphurization of the catalyst under hydrogen. The "ex situ" techniques developed at present use organic polysulphides or sulphur as sulphur-comprising products.
An industrial technique for the presulphurization of catalysts under "ex situ" conditions, based on the use of organic polysulphides of the RS.sub.x R' type (it being possible for R and R' to be identical or different and x.gtoreq.3), has formed the subject-matter of Patent EP 130,850. This process consists in impregnating the catalyst, in oxide form, with a solution of organic polysulphides, such as tert-nonyl polysulphides (TPS 37 or TNPS, sold by Elf Atochem), in a hydrocarbon of the white spirit type. This preliminary stage of incorporation of a sulphur compound of a specific nature in the catalyst is supplemented by a heat treatment of the catalyst in the absence of hydrogen at temperatures not exceeding 150.degree. C. This operation has the effect of removing the organic solvent and of ensuring the attachment of the sulphur to the catalyst by means of the organic polysulphides. At this presulphurization stage, the catalyst is stable in air and can be handled without specific precautions. It is supplied in this state to the user who, after charging to the hydrotreating reactor, can bring the sulphurization of the catalyst to completion under hydrogen for the complete conversion of the metals to metal sulphides.
Other organic polysulphide compounds, with different structures, have also been claimed for the presulphurization of the catalysts under "ex situ" conditions. The products recommended in Patents FR 2,627,104 and EP 329,499 have the general formula: R'--(S.sub.y --R--S.sub.x --R--S.sub.y)--R' and are obtained from olefins and sulphur chloride by a series of successive stages which involve a reaction with an organic monohalide, followed by a reaction with an alkaline polysulphide. In Patent EP 338,897, the products claimed are synthesized from olefins and sulphur chloride with an additional reaction with an alkaline mercaptide or an alkaline polysulphide mercaptate.
The development of a technique for the "ex situ" presulphurization of the catalysts using sulphur in suspension in an oil (U.S. Pat. No. 4,943,547) has presented such problems of industrial application that it has been necessary to develop a new process for sulphurization with sulphur which consists in bringing the catalyst into contact with sulphur and an olefin of high boiling point. The catalyst, thus impregnated, is subsequently heat-treated at a temperature of greater than 150.degree. C. and then the sulphurization of the catalyst is brought to completion under hydrogen at temperatures of greater than 200.degree. C.
Very recently, in Patent FR 2,758,478, it has been shown that the joint use of a tertiary mercaptan and another sulphurization agent, such as dimethyl disulphide, for example, makes it possible to obtain hydrotreating catalysts which are more active with regard to the hydrodesulphurization of hydrocarbon feedstocks than catalysts sulphurized in the absence of tertiary mercaptan.